Net-type spacers are an essential feature in commercially available filtration membrane modules. For example, they are an essential feature of the spirally wound module cartridges used in cross-flow ultrafiltration or in reverse-osmosis separation equipment. Such spacers play a dual role, first, keeping adjacent membrane leaves apart so as to form a feed channel therebetween and, second, promoting the mixing between the bulk of the fluid and the fluid element adjacent to the membrane surface so as to keep the membrane surface relatively clean. Efficient membrane module performance depends on the efficacy of the spacers to increase mass transport away from the membrane surface so as to reduce concentration polarization by enhancing mixing at the membrane surface.
The spacer has a plurality of rows of elongated strands, for example two rows. The strands of a same row are substantially parallel to each other, The rows are disposed in layers where the strands of one row are attached to, typically by fusion, and generally crossing the strands of adjacent rows at an angle.
Several propositions have been made regarding the shape and dimensions of the strands (referred as filaments in some documents) and to the crossing angles of the strands in the mesh (referred as net in some documents),
For example, the use of spacers in the prior art has been mainly as part of the development of membrane modules. For example U.S. Pat. No. 4,861,487, describes that elongated strands of spacers are placed parallel to flow of fluid before making the spirals. In U.S. Pat. No. 5,429,744 a module is made with a spacer material glued to the membrane. In U.S. Pat. No. 4,834,881 a corrugated type of spacer has been described. This type of spacer is claimed to distribute the flow of raw water efficiently. In U.S. Pat. No. 4,902,417 a detailed description of making a spiral wound membrane module is given for application to various types of feeds. The shape of the strands of the spacers is vaguely discussed in this patent document. Also, the description does not mention clearly any specific shapes, and fails to provide any dimensional ratios.
Da Costa et al. (Journal of Membrane Science, 87 (1994) 79-98) have studied effects on pressure drop and flux in a flat sheet membrane system in rectangular cells where a channel between a membrane layer and a top cover is spacer filled. These authors concluded that (FIG. 6 on page 88 of this document): “fluid flows in zig-zag path, changing direction at each mesh”.
Karode and Kumar (Journal of Membrane Science, 193 (2001) 69-84) have estimated pressure drop by solving Navier-Stokes equations in 3D flow domains in similar spacer filled channels and found that observations made by Da Costa et al. were not accurate. Pressure drops and shear rates for various commercially available spacers were also determined with simulations. It was also reported that the dimensions of the strands and angles of intersection of strands in spacers are important parameters. For example, the commercially available spacers included several symmetric and asymmetric spacer designs, the former including two fused together rows of strands crossing each other at an angle, and being of the same plastics material, inter-strand spacing, circular cross-sectional shape and diameter, and the latter differing in its diameter and inter-strand spacing. A further variation in that the two rows of strands cross each other at different angles to each other and to the longitudinal axis of the channel, was also disclosed.